JPS59141570A - Manufacture of melamine - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Urea or its pyrolysis products are treated at atmospheric pressure or at elevated pressures up to about 10 bar as a catalyst and added ammonia or a gas mixture containing ammonia, such as an ammonia/carbon dioxide mixture 1, such as the reaction exhaust gas itself of melamine synthesis. 35 in the presence of
It is known to produce melamine by heating to temperatures between 0 and 450'.

Federal Republic of Germany Patent Publication No. 1204697 >
3, the vaporous melamine is separated from the reaction gas by fractional condensation by cooling the gas to a temperature of 150-250°C. The gas from which melamine has been removed based on vapor tension still contains unreacted urea,
The urea can likewise be washed in a known manner with a melt of urea or a melt of a mixture of urea and biuret, which may optionally also contain further decomposition products of urea, such as cyanuric acid. can. In this case the gas is further cooled. The urea required for this treatment is preferably recycled and brought into direct contact with the reaction gas.

Subsequently, the reaction gas thus cooled is removed from the urea melt by the components recovered from the gas by the melt or by the decomposition products (melamine, isocyanic acid, cyanuric acid, biuret) formed in the melt by heating. etc.) are separated together.

The reaction gas purified and cooled in this way is used as a fluidizing gas for the catalyst in the synthesis reactor and/or as a cooling gas for separating the melamine.

By removing the urea required in the synthesis reactor from this circuit, the by-products and decomposition products recovered from the circuit during the scrubbing of the reaction gas are returned again, so that Il! rate increases. Furthermore, the level of by-products in the urea circuit remains constant and low.

The heat absorbed by the urea conducted in the circuit during cooling and purification of the reaction gas can be used, for example, for melting the fresh urea required for the synthesis and All heat) can be removed by indirect cooling in a cooler.

Federal Republic of Germany Special Specification a''No.2525'781.

A method for extracting heat from a urea melt is described.

The method consists in allowing the cooling of the melt, which hitherto took place exclusively in a cooling tank 81 located outside the urea scrubber, to take place at least partly in the scrubber itself. Therefore,
Cooling of the melt takes place in a cooling element arranged in the urea scrubber, in which case the temperature of the refrigerant is at least 3 °C lower than the temperature of the melt and the melt removed from the urea scrubber has a maximum but.

It should have a humidity of 4°C higher than it had when entering the urea scrubber.

The exhaust gas leaving this scrubber consists primarily of the gaseous components ammonia and carbon dioxide formed simultaneously during the conversion of urea to melamine. This gas should be used because it contains an important component, ammonia.

This mixture can be used, for example, by refluxing the NH2/Co2 mixture through a urea synthesis apparatus, if one is present. Another method for utilizing ammonia contained in exhaust gases consists in reacting ammonia with an acid or an acidic solution, such as nitric acid or sulfuric acid, to obtain ammonium salts. However, such an application necessarily means that urea synthesis is associated with urea production, for example in the field of fertilizer production, where the production of ammonium salts is often undesirable or generally impossible.

In order to free the ammonia from such a bond, it is possible to separate the ammonia from the gas mixture and recover it in pure, preferably liquid form.

Such an example is the method for separating the gas mixture produced in melamine synthesis, as described in German Patent No. 23 of the Federal Republic of Germany.
It was described in 17603 Village and Specification No. 2646804. The gist of the method is to adsorb at least a portion of the gas mixture in a solvent with intimate mixing and with a contact time of at most 0.1 seconds to obtain a solution enriched with respect to ammonia and carbon dioxide. It is in.

When this "absorption" is carried out in two steps, ammonia-free carbon dioxide is sufficiently expelled from the second step. Post-treatment of the now ammonia-enriched solution involves pushing out the ammonia present in the first desorption step in excess of at most a molar ratio of 3 parts ammonia to 1 part carbon dioxide and entraining it from the expelled ammonia. This is done in the form of washing away the carbon dioxide that has been removed. The residual solution is removed from the first desorption column and introduced into a second desorption column operated at a temperature of 87-98 DEG Co'.

In this desorption step, the solution is sufficiently freed of ammonia and carbon dioxide, and the expelled gas mixture is purified with fresh gas mixture fed to an absorption step for further work-up. The bottoms taken out from the second desorption step is again supplied to the absorption step as an absorbent.

Excess water can be drained from the process by stripping a portion of the bottoms of the second desorption step midway through its MK to absorption step in order to achieve volume balance within the entire device. . This bottom liquor still contains some ammonia: t5; (4%, of course, not more than 4%).

The utilization or further treatment of waste water in this manner is not a major drawback in industrial complexes consisting of a large number of different manufacturing plants. This problem is of course important for melamine plants that are not located in locations that meet the above prerequisites.

Accordingly, one object of the present invention is to provide a complete process of the type described at the outset for producing melamine by catalytic reaction of urea and after-treatment of synthetic waste gas from which melamine has been removed, in which the wastewater extracted from the process is substantially It is ammonia-free and does not contain any synthetic sludge decomposed into its constituents ammonia and carbon dioxide, or in each case substantially other constituents, and all of the additional energy required to increase its purity is consumed. The objective was to construct IT14 so that it did not need to be supplied externally to the process and could be sourced from the process itself.

This problem involved introducing the urea-containing melt necessary for cooling the melamine-free gas into the upper and lower parts of the cooling member, and using the bottoms from the second desorption process as the refrigerant. The refrigerant flowing through the cooling member is heated to a temperature of 115-135°C, the heated refrigerant is introduced into a second desorption step, and the bottoms of the second step are maintained at 105-120°C, during which The heated refrigerant has a temperature that is at least 1° C. higher than the respective can crystal.

And, before using the extracted 111% liquid, which is substantially free of ammonia, as a solvent in the 11M process, the bottom liquid is cooled and then 1↓'k 11! It has been found that it is possible to solve this problem by using a gaseous mixture consisting mainly of carbon dioxide that has not been removed for scrubbing.

Based on the present invention ζ, the urea melt is divided into two streams; ir
: ], one of them is forced from above the heat exchanger, and the other is introduced into the urea washer from below, so that the upper part of the urea melt is able to move all the urea melt onto the cooling member. It heats up more intensely than if it were supplied from the source.

As a result, it is possible to exchange heat without increasing the area.
The refrigerant flowing through the tube can be heated to a temperature of 115-135°C. Urea melt division is 150-250' (
: on the basis of the temperature of the tjl' gas entering the urea scrubber and the respective desired degree of 'IMA' that the refrigerant should obtain. The division of the urea melt can be varied within wide ranges. However, the range is limited by the fact that a minimum of 4 people should be placed from the top of the cold 4d1 members so that a sufficient distribution of cooling members for heat transfer is maintained. On the other hand, if too much of the refrigerant is injected from above the cooling member, the temperature rise of the refrigerant will be small.

The amount of heat of the refrigerant heated in this way is used to increase the degree of well-being of the can part in the second desorption step to 105 to 120'c. In order to be able to maintain this humidity, the heated refrigerant should have at least a lo'c fraction/k lower than the respective desired part temperature of the desorption process.

In this case, by increasing the temperature in the second desorption step, it is possible to extract a solution substantially free of ammonia and carbon dioxide.

This 1'11- effluent was cooled to a temperature of 15 to 30'C.
After removal of excess water by synthesis and in accordance with the invention, a reaction mixture is utilized to remove ammonia from a gas consisting mainly of carbon dioxide which has not been absorbed in the absorption step and which now contains ammonia. The solvent is newly introduced as an absorbent into the suction step.

Next, the failure [114] will be explained in detail with reference to the drawings.

The urine 7 is introduced into a container 1, where it is melted and introduced by means of a conduit 2 and a pump 3 into a fluidized reactor 4 in which a catalyst, for example aluminum oxide, is fluidized. It is produced as a fluidizing gas during one reaction.

A gas consisting of ammonia and carbon dioxide from which melamine is separated is used. The gas passes through "-41'J'" and is preheated in a heat exchanger 7 by a gas compressor 6 before being supplied to the fluidized reactor 4. The additional heat required for the reaction is supplied to the catalyst fluidized bed by means of a heating coil 8 into which the heated salt melt is introduced.

The fluidized bed temperature is 350-450°C. The vaporous melamine formed in the fluidized reactor 4 leaves the reactor via the line 9 together with the fluidized gas and the ammonia and carbon dioxide newly formed during the reaction. In the gas-cooled JAJ vessel 10, the gas mixture is cooled until by-products such as melene (but without melamine) are condensed;
The by-product is gas J! It is removed from the reaction gas together with the catalyst dust in the filter 11. The gas purified in this way is introduced via conduit 12 into a condenser 13 in which it is cooled to a temperature of 150 DEG to 250 DEG C. by mixing with cold gas, the origin of which will be explained more fully below. The gas is fed to the condenser via conduit 14. Yorimelamine condenses as the reaction gas cools. The condensed melamine is introduced together with gaseous ammonia and gaseous carbon dioxide.
'15 is passed to a separator 16 where the melamine is separated and removed via conduit 17. The exhaust gas from which solid melamine has been removed is supplied to the cleaning tower 2o via a conduit 19 by a blower 18. In the washing tower, the gas is passed through the vortices 11 and 12.
Washed with urea melt at 0-140°C and cooled at the same time.

The urea melt is removed from vessel l and connected to conduits 2 and 21a.
through the heat exchanger 47 and into the heat exchanger via the conduit 21b. The urea melt heated up to 4° C. is removed from the lower part of the column 2o and is separated into melt and gas in a separator 23. The melt is transferred via conduit 24 to vessel 1, where it uses part of the heat recovered from it to melt the urea newly introduced into the vessel and the other part to generate steam in heat exchanger 22. The steam is discharged to generate 11 and succumbs to heat exchanger 38 to heat column 35, which will be further described below.

The separated gas rI'i at 23 is supplied for three different uses via the conductor-25: a) the flow anti-i no i via the conductor 9','5 k; , to vessel 4.

b) to the condenser 13 via the conductor J14, and C) to the condenser 13 via the conductor H4.
26 and is supplied to the exhaust gas after-treatment described below.

The gas taken out from the conduit 26 enters the absorber 27 at a maximum of 0,1
．． When the γ band was retained for 1 second, it was circulated through the cooler 28 at "1" J.

The solution is brought into contact with a solution at a temperature of -5 to 50° C., preferably 10 to 40° C., in which case the ammonia is absorbed beforehand. 30 and there circulated via the cooler 31, under similar conditions as in the absorber 27.The solution exiting the absorber 30 is brought into contact with
Absorber 27 is fed to conduit 32 . It was removed from the absorber 27 via conduit 33.

The ammonia and carbon dioxide containing solution is heated in a heat exchanger 34 to a temperature of 60-90°C and introduced into a desorption column 35. The upper part of the tower is cooled by a cooler 36. Ammonia is removed from the top of the column via line 37. The can section of the column is heated by a vaporizer 38 to a temperature of 75-90°C. The heat required for this purpose is generated in the heat exchanger 22 and extracted from the steam which is conducted through the conduit 39.

The aqueous solution taken out from the can section of the column 35 and still containing ammonia and carbon dioxide is supplied to the column 41 heated by the bottoms flowing out from the column 41 in a heat exchanger 42 . tower 4
1 can part is 10 at a pressure of 1.2 to 2.0 bar (absolute).
It has a humidity of 5-120°C. The heat required to maintain this temperature is extracted from the circulating effluent of the scrubbing column 20 through conduit 43 and heat exchanger 47. Additionally, any heat required can be supplied by a heat exchanger 45.

Under the operating conditions of column 41 , ammonia and carbon dioxide are completely desorbed into the voids and are incorporated via line 44 into the gas stream fed via line 26 to absorption column 27 . The hot solution now containing less than 0.01% by weight ammonia is routed to conduit 46.
The heat exchanger 42, 34 and 53
The suction 1 in the gas scrubber 49 is cooled to a temperature of 30'.
(This serves to remove any residual ammonia in the C2 gas stream which is discharged from the collector 30 through conduit 48 to 11 as a solubilized solution.) From the column via conduit 50, it is substantially free of ammonia. of carbon dioxide is extracted.

The solution removed from the l'Ir section of column 49 via conduit 51 is fed to absorber 30 as absorption solution.

It was introduced in the gas aftertreatment category. The water, which contains substantially less H3, can be removed via conduit 52.

Example 9.7 tons of urea was melted in container 1 per hour.
via if2 and pump 3'1. It is fed into a fluidized bed reactor 4. Conversion to melamine occurs at 390°C and 1. The reaction is carried out at a pressure of s bar (absolute). A mixture of NH3 and CO2 formed during one reaction is used as the fluidizing gas to keep the catalyst in a fluidized state.

The flowing gas is fed to the reactor 4 via the line 94.5 and by means of the gas compression tank 6 after being preheated to 400'C in the heat exchanger 7. The additional heat required for the reaction reaches the catalyst bed via a heating coil 8 into which the heated salt melt is introduced.

The gaseous melamine formed is allowed to flow out of the tube and cooled to 320°C. Condensed melene and catalyst dust are removed by a gas and toss filter 11. The scum is cooled to 210 DEG C. in a condenser 13 by mixing with gas heated to 140 DEG C. which enters the crystallizer via conduit 14 and crystallizes the melamine. Via the conductor R15, the crystalline melamine is sent together with a gas consisting of ammonia and carbon dioxide to the separation 'd:j1ts where the melamine 3
．． 2tX); 4 separated from the gas via shield '17.

The gas is washed with 1900 t of urea melt having a temperature of 130° C. and flowing into the urea scrubber. In this case, 950 tons will be introduced above the heat exchanger, and the rest will be introduced below the heat exchanger. The melt heated to 134° C. at the bottom of the washer is collected and sent to a new circuit. The bottoms from column 41, which serves as refrigerant, are heated to 128° C. in heat exchanger 47 and the bottoms of the column are heated to 115° C.
(pressure crab 1.7 bar absolute).

The gas stream 44 and the exhaust gas exiting the synthesis section are NH330
00ky, Co23500ky and H20 vapor (
The waste gas is brought into contact with a 11M solution of 200 kp (humidity 140 DEG C.) via four channels 26 in a separator 27 at a temperature of 38 DEG C. and a contact time of approximately 0.02 seconds. The gas mixture that has not been drawn back is brought into contact with the circulating bath liquid again under similar conditions in the second +4V column 30. In the 21st intake 11y, a gas mixture is taken off from the column, which, in addition to CO2, also contains 5.8% by volume of NH3. This gas mixture was removed from column 41 in wash 'a:i49. Rinse with a 1/2 drop solution of substantially pure water. 3500 ky of carbon dioxide is removed from the top of the scrubber, the gas still containing 50 volumes of ammonia 1) plr+bamboo and 11 liters of water vapor.
Contains 0 kg.

The aqueous solution taken out from the absorption tower 27 contains 13.2% of NH3 and 1% by weight of CO2. The solution is transferred to a heat exchanger 34
The reactor is heated to 7100° C. and introduced into a desorption tower 35, where the can section of the tower is maintained at 85°C. The steam required for this purpose is transferred to the heat exchanger 2.
2 via conduit 39. Substantially CO from the top of the tower
3000 kl of 2-free ammonia are removed.

From the can section of column 35, NH35, 2% by weight and CO2
A solution containing 4.2% by weight is taken off. This solution is heated to 110° C. in a heat exchanger 42 and sent from there to a column 41. The vessel section of the column is maintained at 115 DEG C. via the heat exchanger 47 of the washer 20 with an r-+ of 1.7 bar (absolute). NH3 and C0 are completely extruded from the solution in the column.
2 is again supplied to the circuit via conduit 44. A solution containing NH 3
C and carbon dioxide is used as a cleaning solution in column 49 for pre-cleaning. This solution 90 is removed from the thread via line 52.

[Brief explanation of drawings]

The drawing is a system diagram of an apparatus for carrying out the method of the invention. 1... Container, 4... Fluidized bed reactor, 6
・・・・・・Resistance compression, 11・・・Gas filter,
13...Condenser, 16...Separator, 18...Blower, 20...Effluent purification tower, 23...Separator, 27...1st
Absorber, 30... 21st absorber, 35...
... Desorption tower, 41 ... ... tower, 49 ... ... gas 11th generation power
Continuing from page 1 @) Inventor Ernst Jürgen Schiel Federal Republic of Germany 6719 Altreiningen-Gräfenthalstrasse 14 @ Inventor Helmut Grube Federal Republic of Germany 6702 Bad Dürckheim Schlankenkuhler Engineering 37 0 Inventor Martin Maltzandis 3502 Fermal Uhlandstrasse 16 0 Inventor Hein Auer Federal Republic of Germany 6831 Neulsheim Albrecht Thordurer-Strasse 12

Claims (1)

[Claims] A melamine-free synthesis gas is obtained by subjecting urea to a catalytic reaction at high temperatures and condensing melamine from one synthesis gas by fractional distillation. Washing and cooling with a urea-containing melt whose temperature is kept slightly above the melting point, with at least a portion of the heat being drawn off by a cooling element arranged in the urea washer. The temperature of the refrigerant flowing through the cooling element is at least 3° C. lower than the temperature of the melt, and the gas mixture formed in this case, consisting mainly of ammonia and carbon dioxide, is mixed intimately in the solvent for a maximum of 0.1 seconds. by partially absorbing the above gas mixture for a contact time of , separating the resulting solution from the unabsorbed gas consisting mainly of carbon dioxide, and heating the fl-solution containing an excess of ammonia in two steps. Post-treatment is carried out by desorption, in which in the first desorption step the ammonia present in a molar ratio of at most 3 parts of ammonia to 1 part of carbon dioxide is expelled by heating and entrained with the expelled ammonia. The solution taken out from the first desorption step is further heated in a second desorption step to remove ammonia and carbon dioxide, and the solvent-containing gas generated at this time is subjected to a new post-treatment. In a process for the production of melamine, in which melamine is subjected to a new desorption step together with the gas mixture to be removed and the bottoms removed from the second desorption step is used as a solvent in the absorption after removal of excess water. The urea-containing melt necessary for cooling the cooling element is introduced above and below the cooling member, using the bottoms from the second desorption step as the refrigerant and the refrigerant flowing through the cooling member at a concentration of 115 to 135
Heat to a temperature of °C. introducing the heated refrigerant into a second desorption step, in which the can temperature is maintained between 105 and 120 °C, the heated refrigerant having a humidity in each case at least 10 °C higher than the can temperature; and a pre-layer in which the removed bottoms substantially free of ammonia is used as a solvent in the absorption step, and the bottoms are used for scrubbing the gas consisting mainly of carbon dioxide that has not been absorbed after cooling. Characteristic melamine manufacturing method.